1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to elevator systems in which the speed of an elevator car is controlled by a speed pattern generator.
2. Description of the Prior Art
U.S. Pat. No. 3,774,729, which is assigned to the same assignee as the present application, discloses an elevator system in which a speed pattern generator controls the speed of an elevator car by providing a time based speed pattern which accelerates the elevator car to, and then maintains, a predetermined running speed. When the elevator reaches a predetermined position relative to a target floor, the speed pattern generator substitutes a distance based speed pattern for the time based pattern, to control the speed of the elevator car during the slowdown phase of the run. Precise position control at the stopping floor, such as may be provided the shaped magnetic plates and associated transducers disclosed in U.S. Pat. Nos. 3,138,223, 3,207,265 and 3,507,360, which are assigned to the same assignee as the present application, may provide a landing speed pattern which is substituted for the slowdown speed pattern when the elevator reaches a predetermined location relative to the target floor, such as 10 inches from floor level.
In order to provide a high quality ride, without noticeable "bumps" in the elevator car during a run, the transfer from the time based or running speed pattern to the distance based slowdown speed pattern, and the transfer from the distance based slowdown speed pattern to the distance based landing pattern, must be stepless, i.e., the patterns must match at transfer time. Further, in order to provide a comfortable ride, the slowdown must be made at a constant deceleration rate.
An equation may be developed describing a distance based slowdown speed pattern which will start at a specified magnitude at a predetermined distance from the target floor, to cause it to match a like magnitude of the time based running speed pattern, to decelerate at a predetermined constant deceleration rate, and to have a predetermined value, equal to the initial value of the landing speed pattern at the predetermined distance from the target floor at which transfer to the landing speed pattern is to be made. This equation is developed as follows, assuming that the switch or transfer to the landing speed pattern is made 10 inches (0.833 foot) from the target floor: EQU V.sub.x.sup.2 -V.sub.HTAN =2d(X-0.833) (1)
where
V.sub.x =the velocity of the elevator car at a distance X from the target floor PA1 V.sub.HTAN =the required velocity of the elevator car at the distance 0.833 foot from the target floor PA1 d=deceleration rate.
The desired car velocity V.sub.HTAN at 0.833 foot from floor level is known in terms of the desired pattern at 0.833 foot, which will be assumed to be 1.2 feet per second, for purposes of example, the deceleration rate d, and the system time lag T, as follows: EQU V.sub.HTAN =1.2 ft/sec+Td (2)
The distance to floor level will be assumed to be in the form of a digital count, which is converted to a voltage V.sub.d, which will be assumed to be related to the distance to the floor by the following relationship: EQU X=V.sub.D .times.17.6 ft/volt (3)
It will further be assumed that the slowdown speed pattern generator provides a speed pattern DSAN which has the following scale: EQU DSAN=0.333 volt/ft/sec (4)
Combining equations (1) through (4) to relate the required speed pattern voltage DSAN to the distance to go voltage V.sub.D, the deceleration rate d, and the system time lag T, results in: EQU DSAN=.sqroot.L.sub.1 (V.sub.D -L.sub.2)-L.sub.3 ( 5)
where L.sub.1 =3.79d ##EQU1##
In prior art speed pattern generators of which I am aware, Equation (5) was implemented by subtracting L.sub.2 from V.sub.D to provide signal V.sub.D -L.sub.2, multiplying V.sub.D -L.sub.2 by L.sub.1 to provide L.sub.1 (V.sub.D -L.sub.2), taking the square root of L.sub.1 (V.sub.D -L.sub.2) to provide .sqroot.L.sub.1 (V.sub.D -L.sub.2), and then subtracting L.sub.3 to provide .sqroot.L.sub.1 (V.sub.D -L.sub.2)-L.sub.3. The speed pattern generator was designed for a specific deceleration rate, such as 4 ft/sec.sup.2. Provisions for different deceleration rates were made by making the multiplier function programmable with jumper plugs. This, in effect, changed the value of L.sub.1 when a different deceleration rate was selected, and L.sub.2 and L.sub.3 remained unchanged. Thus, Equation (5) was only satisfied when the selected deceleration rate was 4 ft/sec.sup.2, with poor performance at other deceleration rates.
Also, the prior art speed pattern generators require the transfer from the running speed pattern to the slowdown speed pattern to be made at precisely the correct point so that deceleration at the pre-programmed rate will bring the car accurately to the point of the landing pattern substitution. If slowdown is not initiated at exactly the right point, a mismatch between the running speed pattern and the slowdown speed pattern will occur and an undesirable "bump" will be felt in the elevator car at pattern transfer.
If an attempt is made to eliminate this mismatch, such as by developing a calibration voltage just prior to transfer, wherein the slowdown pattern voltage DSAN multiplied by the calibration voltage equals the voltage of the running speed pattern, and by "freezing" the calibration voltage at its value at the precise moment of transfer, an accurate match is made at the transfer point. However, since the distance slowdown pattern DSAN was set equal to the landing pattern at 0.833 ft. from floor level, the calibrated slowdown pattern DSAN is now multiplied by the calibration voltage. This results in a mismatch between the slowdown and landing patterns, and a poor landing. A poor landing is even more objectionable than the original problem, i.e., the mismatch at the higher speed transfer between the running and slowdown patterns.